Bordetella pertussis produces several proteins, in addition to pertussis toxin, with pharmacological activities (toxins). One of these, previously called the dermonecrotic toxin and now designated as heat labilt toxin (HLT), has been isolated in highly purified form. Its structural and biological activities are being characterized. HLT is composed of a single polypeptide chain, molecular weight ca. 150,000 D. the lethal dose in laboratory mice is unusually low, ranging about 10 pg, making it one of the most toxic proteins studied. Monoclonal antibodies have been isolated which confer immunity to intracerebral challenge of mice with B. pertussis providing evidence that HLT might serve as an additional protective antigen. Injection of sublethal doses of HLT induces unusual pathological changes including rapid depletion of the red marrow. B. pertussis produces only trace amounts of HLT, accordingly, DNA recombinant technology must be applied to this problem of production on order to consider the use of this toxin for clinical evaluation.